Conventionally, wells in oil and gas fields are built up with various components: a wellhead housing, a tubing hanger, a casing hanger, etc. Below those components is the drilled borehole for the well. Concentric casing strings are successively installed into the well to reinforce the drilled borehole. These casing strings are typically cemented at their lower ends and sealed with mechanical seal assemblies at their upper ends. Once drilled, the well is typically converted for production by landing a tubing hanger that supports a production tubing string. The production tubing string extends to the reservoir and provides a fluid pathway for directing hydrocarbons to the surface. At the surface, flow of the produced hydrocarbons from the reservoir is controlled by a series of valves that is colloquially called a Christmas tree (or tree).
Traditionally, there are two primary categories of trees—vertical and horizontal—that typically sit on top of the wellhead to control flow. In a vertical tree arrangement, the tree sits on top of a wellhead that supports the tubing hanger and, in turn, the production tubing string. The vertical tree has one or more production bores and contains valves extending vertically to respective lateral production fluid outlet ports in the wall of the tree.
Alternatively, the well may have a horizontal tree arrangement in which the tubing hanger is landed in the tree rather than the wellhead. A lateral production fluid outlet port in the tubing hanger is aligned with a corresponding lateral production port in the tree that leads to a production line, allowing for production through the tubing hanger and into the tree.
Each of these conventional arrangements has certain disadvantages. For example, to remove the production tubing string, which is supported by the tubing hanger landed in the wellhead, in a vertical tree arrangement, the vertical tree would have to first be removed, which can be costly and time consuming. With a horizontal tree, the tubing hanger that supports the production tubing string may be removed without removing the tree. But if it necessary to remove the tree, the tubing hanger and associated production tubing string is also removed.